1. Field of the Invention
The present invention relates to a bag mouth opening device and more particularly to a device that adheres suction members facing each other on either side of a bag conveying path to both sides of the mouths of bags continuously conveyed along the bag conveying path at a constant speed and then moves the suction members away from each other to open the mouths of the bags.
2. Description of the Related Art
FIG. 11 shows the bag mouth opening device described in Japanese Patent Application Laid-Open (Kokai) No. 2002-255119. In this the bag mouth opening device, a pair of suction cups 2, 3 provided so as to face each other on either side of a conveying path (bag conveying path) 1 along which the bags (not shown) are conveyed are continuously rotated along the circular moving paths 4, 5 in mutually opposite directions (see arrows in the circular moving paths 4, 5) at a speed equal to the conveying speed of the bags (see the leftward arrow on the conveying path 1 indicative of the bag conveying direction). The time the suction cups 2, 3 take to make a single rotation along the circular moving paths 4, 5 is adjusted to match the time a bag to be conveyed takes for an inter-bag distance (1 (one) pitch (which is the distance between two bags being conveyed)) along the bag conveying path or the time that is an integer multiple thereof. When the cups 2,3 continuously rotate along the circular moving paths 4, 5, they keep their suction surfaces to be oriented frontally (in other words, toward the bag conveying path 1) at all times while maintaining mutually symmetrical positions on either side of the bag conveying path 1.
In comparison with bag mouth opening devices existing previously, the bag mouth opening device of Japanese Patent Application Laid-Open (Kokai) No. 2002-255119 has such advantages that it is able to offer a simpler and more compact construction, to provide a reduction in vibration and noise, and to increase the speed of operation.
In the bag mouth opening device described in Japanese Patent Application Laid-Open (Kokai) No. 2002-255119, the suction cups 2, 3 are continuously rotated along the circular moving paths 4, 5 in mutually opposite directions at a speed equal to the speed of the bag conveyed (which is a constant speed); and when the cups are closest to each other in the circular moving paths 4, 5, they adhere with suction to both sides of a bag conveyed along the bag conveying path 1. After adhering to the bag, the suction cups 2, 3 travel in the bag conveying direction (toward the left side of FIG. 11) while moving away from each other (away from the bag conveying path 1) as the bag is conveyed.
The suction cups 2, 3 travel along the circular moving paths 4, 5 at a constant speed, and in position P0, where the suction cups 2, 3 come close together again, the direction of travel of the suction cups 2, 3 coincides with the bag conveying direction. Accordingly, in position P0, the speed of travel of the suction cups 2, 3 in the bag conveying direction is equal to the speed of bag conveyed. However, since the suction cups 2, 3 travel along the circular moving paths 4, 5, the speed of travel of the suction cups 2, 3 in the bag conveying direction thereafter becomes subsequently smaller (when compared with the speed of the bag conveyed).
It should be noted that if the speed of bag conveyed (the speed of travel of the suction cups 2, 3 along the circular moving paths 4, 5) is designated as V0, then the traveling speed V of the suction cups 2, 3 in the bag conveying direction after the suction cups 2, 3 have traveled through an angle of θ from the position P0 where the two cups approach toward each other the most along the circular moving paths is shown by V=V0 cos θ.
Although the bag conveying speed V0 is constant, the traveling speed V of the suction cups 2, 3 in the bag conveying direction decreases during the rotation along the circular moving paths 4, 5. After the suction cups 2, 3 adhere to the bag in position P0 (θ=0°), the difference (V0 minus (−) V) between the bag conveying speed V0 and the traveling speed V of the suction cups 2, 3 in the bag conveying direction increases over time, resulting in that the suction cups 2, 3 start lagging behind the bag.
Japanese Patent Application Laid-Open (Kokai) No. 2002-255119 describes in paragraph 13 that the flexibility of the bag absorbs the difference (V0 minus (−) V) between the bag conveying speed V0 and the traveling speed V of the suction cups 2, 3 in the bag conveying direction, so that this speed difference does not lead to any particular problems. However, this description in Japanese Patent Application Laid-Open (Kokai) No. 2002-255119 is based on the premise that bags processed are relatively small in width dimensions. When bags are relatively small in width dimensions, the spacing distance D (see FIG. 11) between the suction cups 2, 3 that have reached the position to fully open the mouth of the bag is small, and as a result the traveling angle θ of the cups from the position P0 along the circular moving paths 4, 5 can be small. For this reason, the speed difference between the bag and the cups does not increase very much, and this speed difference can be absorbed by the flexibility of the bag.
When the bag processed is relatively large in width dimensions, it is necessary to increase the spacing distance D between the suction cups 2, 3 to reach the position to fully open the mouth of the bag. Assuming that the radius of the circular moving paths 4, 5 does not change, then it is necessary to increase the traveling angle θ of the suction cups 2, 3 to fully open the mouth of the bag. If the traveling angle θ of the suction cups 2, 3 increases, the traveling speed V of the suction cups 2, 3 in the bag conveying direction becomes smaller, and the speed difference (V0−V) between the bag conveying speed V0 of the bag and the traveling speed V of the suction cups 2, 3 in the bag conveying direction becomes larger. For this reason, positional misalignment between the bag and the suction cups 2, 3 in the bag conveying direction increases as much as it becomes difficult to absorb the misalignment even if the advantage of the flexibility of the bag is taken into account, resulting in that the suction cups 2, 3 become detached from the bag while the mouth is opened, causing mouth opening failures. In addition, even in a case that the suction cups 2, 3 do not become detached from the bag, since forces in a direction opposite to the conveying direction act on the bag while the mouth is being opened, various problems would arise, including that the bag is detached from the grippers, the bag is displaced from the regular holding position, and the shape of the opened bag mouth is distorted.
If the radius of the circular moving paths 4, 5 in the above-described bag mouth opening device can be increased, even if the traveling angle θ of the suction cups 2, 3 reached the position where the mouth of the bag is fully opened is small, the spacing distance D between the suction cups 2, 3 can be increased, and the speed difference between the bag conveying speed V0 and the traveling speed V of the suction cups 2, 3 in the bag conveying direction does not become excessively large even when the mouth of bag that is relatively large in width dimensions is opened, and this speed difference can be absorbed by taking (advantage of) the flexibility of the bag into account. However, in the above-described bag mouth opening device, the speed of rotation of the suction cups 2, 3 along the circular moving paths 4, 5 is adjusted to match the conveying speed of bag, and the time period the suction cups 2, 3 take to make a single rotation along the circular moving paths 4, 5 is adjusted to match the time the bag is conveyed for an inter-bag distance (1 pitch), or it is set to an integer multiple thereof. For this reason, the radius of the circular moving paths 4, 5 is inevitably set to a constant value. In other words, in the above-described bag mouth opening device, it is substantially difficult to vary the radius of the circular moving paths 4, 5 in accordance with the width dimensions of the bags to be processed.